Many locations utilize a high pressure (transmission) network and a lower pressure (distribution) network to supply natural gas through a local area. The transmission network typically acts as a freeway to economically send the gas over long distances to the general area, while the distribution network acts as the roads to send the gas to the individual users within a local area. Pressures of these networks vary by location, but typical values are between 30-80 bara for transmission and 3-20 bara for distribution. Some applications (e.g., cogeneration, boilers, etc . . . ) have high flowrates of natural gas and other utilities such as nitrogen, which are letdown to the consumer or to the lower pressure network at relatively constant flow, pressure and temperature conditions. This pressure letdown energy is often not utilized.
Traditionally, natural gas is compressed and sent through pipelines under high pressure to transport the gas to customers. High pressures are used in order to reduce the volumetric flow of the gas thereby reducing pipe diameters (capex) and/or compression energy related to pressure losses (opex). Pipeline operators also utilize the high pressure as a buffer to accommodate transient demands. When the gas has arrived at its use point, the pressure of the natural gas is reduced in one or more control valves to its final pressure for consumption. The available energy from the reduction in pressure of the natural gas is wasted in the control valves as well as any chilling effect (also known as the Joule Thomson effect) caused by the flow of natural gas through these devices. Additionally, such systems often require heaters and condensate systems due to the colder conditions of the downstream gas.
In the past, advantage has been taken of this wasted energy by facilities utilizing the energy and refrigeration effect of expanding the natural gas. One such facility employed a natural gas pressure reduction station (“Letdown Station”) to make liquefied natural gas (“LNG”) or liquid nitrogen (“LIN”). A majority of the natural gas entering the plant under high pressure from the transportation pipeline was cooled and sent to an expansion turbine where energy and refrigeration were generated. The remainder of the stream was subsequently cooled with the refrigeration and a portion liquefied. The liquefied portion was then passed to a storage tank as LNG product. The natural gas that was not liquefied was warmed, collected and sent to the low pressure header at a lower pressure than the high-pressure header.
U.S. Pat. No. 6,196,021 describes a system that uses natural gas expansion to provide refrigeration to liquefy a natural gas stream, which is then vaporized by heat exchange with a nitrogen stream to cool the nitrogen stream. This refrigeration supplements refrigeration provided by nitrogen pressure letdown and a nitrogen cycle to provide liquid nitrogen.
Similarly, U.S. Pat. No. 6,131,407 describes a system that produces LIN to be sent directly to an air separation unit (“ASU”) to assist refrigeration of the ASU. U.S. Patent Application Publication No. 2014/0352353 describes a similar system to the system of disclosed by U.S. Pat. No. 6,131,407, but adds that the LIN produced can be sent to a tank instead of being used to liquid assist the ASU. In each of these systems, the produced LNG is revaporized in order to provide cooling for the production of liquid nitrogen.
U.S. Pat. No. 6,694,774 describes a system that uses natural gas letdown to provide refrigeration to produce a liquefied natural gas stream, where the refrigeration is supplemented by a closed loop mixed refrigerant cycle. Expansion of the pressurized natural gas provides the “high temperature” cooling and the mixed gas refrigerant cycle provides the low temperature cooling for liquefaction of a second portion of the natural gas. The primary point of emphasis in '774 was to power the compressor of the refrigeration cycle using work generated by the expansion of the pressurized natural gas stream. However, in embodiments in which the gas to be liquefied must be compressed prior to liquefaction, the power used to run the compressor is provided by an electric motor.
Therefore, it would be advantageous to provide a method and apparatus that operated in a more efficient manner yielding a lower cost of LNG.